Photochemical Vapor Deposition Method Research Articles

Dependence of hydrogen and oxygen incorporation on deposition parameters in photochemical vapor deposited mercury free silicon nitride films

Silicon nitride films have been deposited on p-type Si (100) by mercury-sensitized photo-chemical vapor deposition (photo-CVD) method varying deposition pressure and substrate temperature. Energy dispersive X-ray fluorescence spectra of the samples show that the incorporation of mercury in the films, if any, is below 20 ppm. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy studies show the incorporation of oxygen and hydrogen in all the films, which is a function of the deposition parameters. Higher substrate temperature favors the formation of SiH bonds and reverse is the case for the formation of SiNH bonds. The sample deposited at low temperature (170 °C) shows the presence of less unreacted silicon (4%) in comparison to the sample (12.5% unreacted silicon) deposited at higher deposition temperature (250 °C), but the variation of pressure shows no significant change in terms of the unreacted silicon. The incorporated hydrogen and oxygen passivate surface defects thereby influencing interface electronic state densities ( D it) and fixed insulating charges ( Q ss).

Electrical properties of photo-CVD boron-doped hydrogenated nanocrystalline silicon-carbide (p-nc-SiC:H) films for uncooled IR bolometer applications

Electrical properties of boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) films grown by mercury-sensitized photo-chemical vapor deposition method have been investigated for uncooled IR bolometer applications. The temperature coefficient of resistance (TCR), conductivity, and 1/ f noise characteristics have been measured and the detectivity of a bolometer has been estimated from them. It has been observed that as ethylene gas flow ratio (C 2H 4/SiH 4) increases, TCR of films increases due to larger optical bandgap. However, the 1/ f noise also increases due to the reduction of the number of total carriers in the films for higher activation energy ( E a). The maximum TCR of 2.3%/K has been measured for the films deposited with an ethylene gas flow ratio of 0.47, while the maximum detectivity has been estimated for those with an ethylene gas flow ratio of 0.07 because of the lowest 1/ f noise.

Highly conductive boron-doped nanocrystalline silicon-carbide film prepared by low-hydrogen-dilution photo-CVD method using ethylene as a carbon source

Boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) films with a low-concentration of hydrogen-dilution were grown by a mercury-sensitized photo-chemical vapor deposition method using silane (SiH 4), hydrogen (H 2), diborane (B 2H 6), and ethylene (C 2H 4) as a carbon source. From the Raman and FTIR spectrum measurements, the p-nc-SiC:H film is composed of nanosize crystal silicon embedded in a hydrogenated amorphous silicon-carbide matrix. A dark conductivity as high as 1.7×10 −1 S/cm, with an optical bandgap is 2.0 eV, and a crystal volume fraction of 50%, were obtained. We tested these films as window material for amorphous silicon solar cells, obtaining an initial conversion efficiency of 10.4% without using any back reflectors.

Light-annealing effect on a-Si:H photodiode with the Ar laser-irradiation

The light stability of a-Si:H photodiode is much improved by light-annealing treatment, which is produced by the Hg-sensitized photochemical vapor deposition method. The dark-current density of the photodiode with light-annealing (2 h, 180 °C) is about seven times smaller than that without light-annealing after Ar laser irradiation ( λ=514 nm , ∼6 W/cm 2, 7 min). On the other hand, the photodegradation property of the photocurrent is not improved by light-annealing for a-Si:H single layer. Although the defects are produced by light-annealing around the mid-gap level of a-Si:H(i) film, the a-Si:H(i)/a-SiC:H(p) boundary of the photodiode is considered to be improved in the case. The electric field of a-Si:H(i) is considered to become strong, which decreases the dark-current density of the photodiode after Ar laser exposure.

Aluminum-seeded growth of microcrystalline Si thin film onto SnO2 substrate

To improve the crystallinity of thin boron-doped microcrystalline Si (p-μc-Si:H) films at an initial growth stage, we tried an Al-seeding method, which is to deposit an ultrathin (∼50 Å) Al layer onto a transparent conducting oxide before preparing the p-μc-Si:H film by using a mercury-sensitized photochemical vapor deposition method. It was found that the Al layer serves as seeds to facilitate the p-μc-Si:H with high crystallinity at an initial growth stage and the Al seeds stimulate the nucleation of Si crystallites. It was also found that the absorption loss in the ultrathin (∼50 Å) Al-seed layer is negligible in the visible wavelength region due to its natural oxidation. When used in superstrate-type p-i-n amorphous silicon solar cells, even the Al-seeded p-μc-Si:H film having a several hundred angstrom thickness acts as an excellent p layer compared to the conventional p-μc-Si:H.

Comparison of gas-phase reactions in low-temperature growth of Si films by photochemical vapor deposition and the hot wire cell method

Mercury sensitized photochemical vapor deposition and hot wire cell method were studied as growth techniques of Si films at low temperature (<400°C). In both techniques, the boundaries indicating the structural transition of the Si films were observed to depend on the substrate temperature and the total pressure. The results of the theoretical analysis indicated that the film structure is correlated with the ratio of atomic hydrogen to SiH 3 radical on the growing surface. By the hot wire method, polycrystalline Si films were successfully obtained on glass substrate without H 2 dilution. These films had a growth rate >1 nm/s, which was about 10 times larger than that of the films grown by photochemical vapor deposition. This method can generate a larger flux of SiH 3 and the flux of H is also large enough to induce crystallization on glass substrate.

The optical and electrical studies of hydrogen passivation in GaInP/GaAs heterostructures

It is shown that hydrogen passivation by the photochemical vapor deposition method can have a significant influence on GaInP/GaAs heterostructures. The effect has been investigated by low-temperature photoluminescence and current–voltage and capacitance–voltage experiments. The photoluminescence measurement shows a strong increase in the luminescence intensity after hydrogenation. It is interpreted in terms of the passivation of nonradiative recombination defect centers by atomic hydrogen. The effect is also accompanied by a simultaneous decrease in the carrier concentration as shown from the capacitance–voltage measurements. In addition, the effect of hydrogenation is confirmed by the improvement of the Schottky-diode properties. These results provide concrete evidence to support the passivation of impurities and defects by atomic hydrogen in GaInP/GaAs heterostructures.

SIO 2 thin film preparation using dielectric barrier discharge-driven excimer lamps

By using a photochemical vapor deposition method with a Xe 2 excimer lamp (172 nm, 20 mW cm −2 output power), silica films have been prepared by means of a single precursor process from Tetraethoxyorthosilicate (TEOS) at room temperature. Transparent thin films of SiO 2 were obtained on sapphire and quartz single crystal substrates with a deposition rate of 0.9 nm min −1. They were mainly composed of amorphous SiO 2, although small amounts of residual organic materials were contained. The refractive index was 1.476 at 632.8 nm. The surface roughness decreased with the film thickness and reached 0.2 nm-rms. These findings indicate that the VUV excimer lamp CVD is a promising method for preparing smooth, dense and fine thickness-controllable films of SiO 2 at room temperature.

Boron-doped amorphous diamondlike carbon as a new p-type window material in amorphous silicon p-i-n solar cells

A boron-doped hydrogenated amorphous diamondlike carbon (a-DLC:H) was prepared using a mercury-sensitized photochemical vapor deposition (photo-CVD) method. The source gases were B2H6 and C2H4. By increasing the boron doping ratio (B2H6/C2H4) from 0 to 12 000 ppm, the dark conductivity increased from ∼10−9 to ∼10−7 S/cm. A boron-doped a-DLC:H with an energy band gap of 3.8 eV and a dark conductivity of 1.3×10−8 S/cm was obtained at a doping ratio of 3600 ppm. By using this film, amorphous silicon (a-Si) solar cells with a novel p-a-DLC:H/p-a-SiC double p-layer structure were fabricated using the photo-CVD method and the cell photovoltaic characteristics were investigated as a function of a-DLC:H layer thickness. The open circuit voltage increased from 0.766 V for the conventional cell with a 40-Å-thick p-a-SiC to 0.865 V for the cell with a p-a-DLC:H (15 Å)/p-a-SiC (40 Å) double p-layer structure. The thin (&amp;lt;15 Å) p-a-DLC:H layer proved to be an excellent hole emitter as a wide band gap window layer.

Post Hydrogen Treatment Effects of Boron-doped a-SiC:H p-Layer of a-Si:H Solar Cell Using a Mercury-Sensitized Photo-Chemical Vapor Deposition Method

Post hydrogen treatment effects of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p-layer of p/i/n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated by measuring the thickness, electrical, and optical properties of the film before and after hydrogen treatment. It was found that the boron-doped a-SiC:H film was simultaneously etched and passivated by the treatment. The performance of the cell with a hydrogen treated p-layer was improved by ∼7% due to an increase in open circuit voltage (V oc) and fill factor (F.F.), compared to that of the untreated cell, although the p-layer thickness was nearly identical in both cases. The increase in V oc and F.F. could be explained by an increase in the built-in potential due to a decrease in the film activation energy. This electrical property improvement was well explained by the passivation effect of a SiH2/SiH ratio decrease of the film calculated by FTIR spectra. These film changes by post hydrogen treatment are considered to occur in the bulk of the boron-doped a-SiC:H film.

Etching and passivation effects on boron-doped amorphous silicon carbide p layer of amorphous silicon solar cell by hydrogen treatment using a mercury-sensitized photochemical vapor deposition method

Etching and passivation effects of hydrogen treatment of boron-doped hydrogenated amorphous silicon carbide (a-SiC:H) film used as a p layer of p-i-n type amorphous silicon based solar cells using a mercury-sensitized photochemical vapor deposition method were investigated. For the hydrogen treatment of the p-layer film, longer p-layer deposition time was needed to obtain the same thickness as for no hydrogen treatment because of hydrogen etching effect. However, the cell performance was improved by ∼7% due to an increase in the open circuit voltage (Voc) and fill factor (FF) although the p-layer thickness was nearly identical in both cases. The increase in the Voc and FF could be explained by an increase in the built-in potential due to a decrease in the film activation energy. Moreover, the electrical property improvement of the film was well explained by the passivation effect of a SiH2/SiH ratio decrease and a hydrogen content increase calculated from Fourier transformed infrared absorption measurements.

Preparation of High-Quality Undoped Microcrystalline Silicon with High Deposition Rate Using Mercury-Sensitized Photochemical Vapor Deposition Method

We prepared undoped microcrystalline silicon films (µ c- Si:H) using a mercury-sensitized photochemical vapor deposition system under various film deposition conditions of SiH4/(SiH4+H2) gas ratio and total gas flow rate, and investigated the dependence of the film quality on the deposition conditions in order to obtain high-quality undoped microcrystalline films at a high deposition rate. Up to a SiH4/(SiH4+H2) gas ratio of ∼0.1, the crystallite size of the µ c- Si:H films decreased linearly as the ratio increased, but the volume fraction of crystallites remained constant at about 80%. The dark conductivity of the films was very low, in the range of 10-7–10-6 Scm-1 under nearly all experimental conditions, and so a high photosensitivity of ∼550 was obtained without the addition of a dopant gas such as diborane. In addition, a high deposition rate of ∼30 Å/min was obtained at a low total flow rate of 21 sccm. No degradation of the microcrystallinity was observed for the low total flow rate.

Dramatic improvement of performance of visible hydrogenated amorphous silicon carbide based p-i-n thin-film light-emitting diodes by two-step hydrogenation

The effects of two-step hydrogenation on the performance of visible p-i-n thin-film light-emitting diode (TFLED) have been investigated. The TFLEDs were fabricated by a photochemical vapor deposition (CVD) method. A hydrogenation process was performed in two steps: One was an in situ hydrogenation process using a photo-CVD system and the other an ex situ hydrogenation process using a plasma apparatus after TFLED fabrication. It was found that the performance of visible a-SiC:H-based p-i-n TFLEDs was drastically improved by a two-step hydrogenation process. The threshold voltage decreased by about 3 V, the electroluminescence peak shifted towards a shorter wavelength, from 680 to 590 nm, and the brightness increased from 1.3 to 128 cd/m2.

Characteristics of a-Si:H films prepared by a novel laminar flow photo-chemical vapor deposition method

The a-Si:H films prepared by a laminar flow photo-chemical vapor deposition method were investigated. The SiH 2 bond content ( C SiH 2 ) in the a-Si:H film and the relative residual current increased along the gas flow direction. The increases in C SiH 2 and in the residual current are due to the increase in SiH 2 density. The SiH 2 is produced by a reaction between SiH 3 radicals during transport.

In situ hydrogenation of visible a-SiC:H-based p - i - n type thin-film light-emitting diodes using the photochemical vapour deposition method

The effects of in situ hydrogenation on the performance of visible a-SiC:H-based p - i - n type thin-film light-emitting diodes (TFLEDs) have been investigated. The TFLEDs were fabricated with a photochemical vapour deposition method. In the process of TFLED fabrication, with a stream of of 50 sccm and ultraviolet light irradiation at a substrate temperature of and a process pressure of 0.3 Torr for 30 min, in situ hydrogenation was performed at the base of a p layer with various thicknesses of i-a-SiC:H layers (0, 10, 20 and 30 nm). It was found that the performance of visible TFLEDs was dramatically improved by in situ hydrogenation. With in situ hydrogenation after deposition of the 30 nm thick luminescently active i-a-SiC:H layer, the threshold voltage was decreased by about 2 V, the electroluminescence peak shifted from 680 nm to 625 nm and the brightness increased from to .

Hydrogen passivation of visible p-i-n type thin-film light-emitting diodes

The effects of hydrogen passivation on the performance of visible p-i-n thin-film light-emitting diodes (TFLEDs) have been investigated. The TFLEDs were fabricated using a photochemical vapor deposition method. The hydrogenation process was performed using an inductively coupled plasma system at a rf power of 800 W and a process pressure of 20 mTorr for 30 min. It was found that hydrogenation dramatically improved the performance of these TFLEDs. The threshold voltage was decreased by about 1 V, the electroluminescence (EL) peak shifted from 704.5 to 689 nm, the EL intensity increased by a factor of 3, and the brightness increased from 1 to 24 cd/m2 by 24 times.

The dependence of etch rate of photo-CVD silicon nitride films on NH 4F content in buffered HF

The influence was studied of the concentration of ammonium fluoride (NH 4F) on the etch rates of silicon nitride films deposited by the mercury-sensitized photochemical vapour deposition method. The composition of the buffered HF was varied between 0 and 40 weight percent (wt%) NH 4F with 2 to 12wt% hydrofluoric acid (HF). The etch rates as a function of buffered HF composition were measured for films deposited under various process parameters, viz. reactant gas ratio, substrate temperature and chamber pressure. The results of etch rates as a function of process parameters were correlated to variations in material density and silicon content (Si/N ratio) in the films.

Hg-Sensitized Photochemical Vapor Deposition Application to Hydrogenated Amorphous Silicon Photoconversion Layer Overlaid on Charge Coupled Device

An Hg-sensitized photochemical vapor deposition method has been developed which has enabled a hydrogenated amorphous silicon photoconversion layer to be overlaid on a charge coupled device (CCD) imager, without a pixel separation structure. This chemical vapor deposition (CVD) method has been used to realize imaging devices with high sensitivity and high resolution for high-definition TV.

Improvement of a-Si Solar Cell Fabricated by Mercury-Sensitized Photochemical Vapor Deposition Using H2 Dilution Technique

The fabrication of hydrogen-diluted amorphous silicon (a-Si) film by the mercury-sensitized photochemical vapor deposition (photo-CVD) method has been performed. The hydrogen content in the film along with the optical bandgap was found to increase with the hydrogen/silane dilution ratio, while the dihydride/monohydride bond ratio decreased at the substrate temperature of 150° C. As a result, a decrease in the defect density was observed. Moreover, the photo- and dark conductivities were maintained in the range of 3×10-5 and 1×10-10 S/cm, respectively, in spite of the fact that the optical bandgap widened. The solar cells with hydrogen diluted i-layers were fabricated and showed improvement in the open-circuit voltage and fill factor (FF), resulting in the increase of both initial and stabilized efficiencies. The forward- and reverse-bias dark I-V characteristics measurement of solar cells revealed improvement in quality and stability of the hydrogen-diluted i-layers.

Tin dioxide thin films prepared by photochemical vapour deposition from tin(II) acetate

Tin dioxide thin films were prepared by a direct photochemical vapour deposition method. The raw material was tin(II) acetate. A 6W low-pressure mercury lamp was used as a light source. At a substrate temperature above 150°C, the thin film was obtained in air with a deposition rate which is higher than that for thermal chemical vapour deposition. UV irradiation without premixing oxygen gas improves crystallinity, suppressing the intrusion of carbon contamination.